1. Field of the Invention
The present invention is related, in general, to avoiding multiple detection of the same random access channel (RACH) preamble in a wireless communication system.
2. Description of the Related Art
Expanded efforts are underway to support the evolution of the Universal Mobile Telecommunications System (UMTS) standard, which describes a network infrastructure implementing a next generation Wideband Code Division Multiple Access (W-CDMA) air interface technology. A UMTS typically includes a radio access network, referred to as a UMTS terrestrial radio access network (UTRAN). The UTRAN may interface with a variety of separate core networks (CN). The core networks in turn may communicate with other external networks (ISDN/PSDN, etc.) to pass information to and from a plurality of wireless users, or user equipments (UEs), that are served by radio network controllers (RNCs) and base transceiver stations (BTSs, also referred to as Node Bs), within the UTRAN, for example.
Standards related to UMTS have introduced several technologies in an effort to ensure that any associated control and/or data information is carried in an efficient manner, in an effort to improve overall system capacity. One set of issues being addressed by the 3rd Generation Partnership Project (3GPP), a body which drafts technical specifications for the UMTS standard and other cellular technologies, includes considerations related to a start-up phase of a given UMTS network.
During a start-up phase for a UMTS network, there may be only a small number of users. However, it may be desirable for the cost expensive antenna installation to be done a single time, and that an upgrade of the Node-B towards a higher number of supported users be done by hardware alone, upgrading the Node-B itself.
One possible configuration that may facilitate the above is referred to as a ‘pseudo omni cell’. A pseudo omni cell employs the conventional three sector configuration of a Node-B, but employs it as a single cell configuration consisting of three sectors, where each sector may represent a geographical area that is covered by an antenna of a Node B. This concept is referred to as Multiple Receive Pseudo Omni (MRPO). With increasing numbers of UEs, the Node-B may be up-graded from a 3-sector/1-cell configuration to a 1-sector/3-cell configuration without expensive changing of the deployed antenna configuration.
However, the above approach may pose certain problems, particularly with regard to detection of random access channels (RACHs) which are transport channels carrying data mapped from upper level logical channels (OSI Layers 3-7). The RACH transport channels may then be sent by the UE in the uplink to the Node B over physical channels such as a physical RACH (PRACH).
In general, physical channels are defined by a specific carrier frequency, scrambling code, channelization code (optional), time start & stop (giving a duration) and, on the uplink, relative phase (0 or π/2). Time durations may be defined by start and stop instants, measured in integer multiples of chips. Suitable multiples of chips may be based on a radio frame, slot and/or sub-frame configuration. A radio frame is a processing duration which consists of 15 slots. The length of a radio frame corresponds to 38400 chips. A slot is a duration which consists of fields containing bits. The length of a slot corresponds to 2560 chips.
The default time duration for a physical channel may be continuous from the instant when it is started to the instant when it is stopped. Transport channels may be characterized as being capable of being mapped to physical channels. Within the physical layer itself, the exact mapping may be from a composite coded transport channel (CCTrCH) to the data part of a physical channel. In addition to data parts there also exist channel control parts and physical signals.
To understand the potential problems with RACH detection in an MRPO sector-cell configuration of a communication network such as a UMTS network, aspects of the conventional RACH structure and operation are described in further detail below.
As discussed above, the PRACH is used to carry the RACH, where the RACH is an uplink transport channel intended to be used to carry control information from the user equipment (UE), such as requests to set-up a connection, as part of a random-access transmission in the uplink from a UE to a Node B, for example. The RACH may also be used to send small amounts of packet data from the UE to the network.
The random-access transmission is based on what is referred to as a Slotted ALOHA approach with fast acquisition indication. A UE can start the random-access transmission at the beginning of a number of given time intervals, which are referred to as ‘access slots’. There are 15 access slots per two frames, typically one frame of 8 slots and a second frame of seven slots, and are spaced 5120 chips apart, as specified in 3GPP TS 25.211, V6.0.0 (2003-12), entitled “Physical channels and mapping of transport channels onto physical channels (FDD)”. The timing of the access slots and the acquisition indication is described in TS 25.211, section 7.3. Information on what access slots are available for random-access transmission may be given by higher layers (i.e., OSI layers 3-7). A UE sends a series of PRACH preambles. A PRACH preamble is may be answered by the Node B using an Acquisition Indication Channel (AICH) in the downlink. The Node sends either a ACK or a NACK back to the mobile. If a ACK is received by the mobile it starts the transmission of the RACH message.
For proper system operation, the random-access transmission (e.g., the RACH message) should be able to be received from the entire desired cell coverage area. The AICH is a fixed rate (SF=256) physical channel used to carry Acquisition Indicators (AI), where AIs corresponds to signature s on the PRACH. The AICH consists of a repeated sequence of 15 consecutive access slots, each of length 5120 chips. Each access slot consists of two parts, an AI part consisting of 32 real-valued signals a0, . . . , a31, and a part of duration 1024 chips with no transmission that is not formally part of the AICH. Thus, in order to support data transfer in the uplink and downlink directions, the RACH transport channel utilizes two physical channels, the PRACH in the uplink and the AICH in the downlink. The AICH uses a reserved OVSF channelization code to indicate this. The code number is indicated to the UE with the system information on BCH. The AICH uses the downlink primary scrambling code of the cell.
A PRACH may be described by the following attributes: Preamble Scrambling Code, Preamble Signatures, Sub-channels and Uplink Message Channelization Code. The random-access transmission consists of one or several preambles of length 4096 chips and a message of length 10 ms or 20 ms.
There are a total 8192 PRACH preamble scrambling codes. These codes are divided into 512 groups with 16 codes in each group. The RNC uses Radio Resource Control (RRC) System Information Broadcast message numbers 5 and 6 to broadcast the actual used PRACH scrambling code group. There may be a one-to-one mapping between the group of PRACH preamble scrambling codes in a cell and the primary scrambling code used in the downlink of a cell, as shown in expression (1) below:PRACH Preamble Scrambling Code=16*Downlink Scrambling Code+PRACH Scrambling Code Group   (1)
There are a total of 512 downlink scrambling codes available for one cell. The code that is used may be determined by the UE during a cell search procedure. The relationship between the downlink and the uplink scrambling codes is such that the same code allocation scheme may be applied on both the downlink and uplink.
As noted above, the PRACH preamble consists of 4096 chips, which is a sequence of 256 repetitions of Hadamard codes of length 16. The preamble is scrambled with a scrambling code. All 16 Hadamard codes can be used for random access. The RNC uses the RRC System Information Broadcast messages numbers 5 and 6 to broadcast the “PRACH system information list”, providing the actual used PRACH preamble signatures and the access slots that are used by UEs in a given cell. The Hadamard codes may be referred to as signature of the preamble or “preamble signature”. Because of its orthogonality, several access attempts with different preamble signatures may be simultaneously detected.
Uplink channelization codes of the RACH message are derived as follows. Each of the 16 available preamble signatures points to one of the 16 nodes in the Orthogonal Variable Spreading Factor (OVSF) code-tree that correspond to channelization codes of length 16. The sub-tree below the specified node is used for spreading of the message part.
A RACH sub-channel defines a sub-set of the total set of uplink access slots. There are a total of 12 RACH sub-channels (sub-channels 0-11), which describe the access of the following uplink access slots, shown in Table 1 below:
TABLE 1Available uplink slots for different RACH sub-channelsSFN mod 8 ofcorrespondingP-CCPCHSub-channel numberframe012345678910110012345671121314891011201234567391011121314846701234558910111213146345670127891011121314
Referring to Table 1, there is shown 12 RACH sub-channels (0-11). Additionally, there is shown 15 access slots (0-14) per two frames (System Frame Numbers (SFNs). Each row indicates which access slots are in which frame, each column indicates what access slots are for each sub-channel.
For example, access slots 0-14 are provided across SFNs 0 and 1, 2 and 3, etc. The understand Table 1, each sub-channel is assigned specific access slots in a given frame. Thus for example, the access slot for sub-channel 1 in frame 0 is access slot 1. Sub-channel 2 is assigned access slots 2, 14, 11, 8 and 5 over the 8 SFNs. A UE may use any access slot that is available based on the broadcast received from the RNC, and may use any preamble signature indicated available by the broadcast.
FIG. 1 illustrates a conventional periodic sub-channel structure for a RACH. FIG. 1 should be viewed with occasional reference to Table 1. As shown in FIG. 1, there is a periodic structure for the RACH sub-channel. The left-handle side (Y-axis) illustrates the sub-channels, and the x-axis illustrates the two frames containing the 15 access slots (slots 0-14) as discussed with respect to Table 1, here shown as SFN 0 (8 access slots) and SFN 1 (7 access slots).
For RACH timing, a preamble-to-preamble distance may depend on a AICH transmission timing parameter. This parameter may be assigned by the RNC RRC layer and is broadcast to the in the system information block 5. The minimum preamble-to-preamble distance may be three (3) access slots (if the AICH transmission timing parameter is set to 0), or four (4) access slots if the AICH transmission timing parameter is set to 1.
If the Node B detects a RACH preamble in an RACH access slot with a certain signature, it echoes this signature in the associated AICH access slot multiplied with an AI. The AI is used to indicate, to the UE, that its RACH access request has been accepted (or not).
The PRACH resources (i.e., access slots and preamble signatures) may be divided between different Access Service Classes (ASCs) in order to provide different priorities of RACH usage. In the case where multiple PRACHs use the same PRACH preamble scrambling code, the available preamble signatures per PRACH should be disjunct, i.e., each PRACH should be using a set of preambles which are not used by the other PRACH. For a multiple PRACH per cell configuration with different PRACH preamble scrambling codes, no special cell configuration is necessary, except that suitable ASC usage and RACH partitioning should be employed.
FIG. 2 illustrates a timing diagram of a conventional channel element for RACH preamble detection. A baseband unit (BBU) is a device that can process several transport/physical channels in parallel. A channel element (CE) may be understood as the functional unit or device for handling a single transport/physical channel. A BBU may have multiple CEs. A BBU may be an element or device of a base station such as a Node B.
Referring to FIG. 2, there is shown a preamble part and a message part for a RACH for two UEs, UE1 and UE2 across four (4) SFNs. A CE of a Node B in a given cell (such as for UE1) looks for the preamble, here shown in slot 0 of SFN 0.
In general, if the CE detects the preamble, the CE determines whether it understands the preamble signature of the preamble. If the preamble signature is understood, the CE then determines whether it has an available message demodulator with free capacity (i.e., a demodulator or receiver that has no on-going reception) and if so, the CE prepares the demodulator with the appropriate channelization code according to the signature and sends a positive message (ACK) on an AICH to UE1. Based on receiving the ACK on the AICH, UE1 transmits the message part of the RACH to the CE, which forwards the request to the controlling RNC in the UTRAN, for example such hat the RNC may allocate a channel for the call.
Thus, the CE has a single RACH preamble detector. The RACH preamble detector is configured with the PRACH scrambling code and the set of allowed preamble signatures. The CE may further include a message demodulator as discussed above (not shown) for the PRACH and an AI generator (not shown). The CE preamble detector is able to detect preambles of up to 16 signatures.
Conventionally, the CE selects the strongest received preamble and sends for its signature a positive AI (ACK AICH) if the message demodulator is idle, or a negative AI (NACK AICH), if the message demodulator is busy. Per each access slot one or more AI may be sent. A UE might not get an AI with their signature in response to a sent preamble. These UEs may retry the access and send another preamble in the next possible access slot with a new signature.
As discussed above, the basic concept for Multiple Receive Pseudo Omni (MRPO) is that the RNC views a 3-sectorized Node-B as an omni-cell Node-B. One logical cell and one set of common channels are created by the RNC via the Node B Application Part (NBAP), which is a control plane protocol used at the luB interface to carry signaling traffic to manage the logical resources at the Node B. In the uplink, the signals of the three sectors have to be combined in the CE.
As mentioned above, only one preamble detector is included in a given CE. In other words, one CE is necessary to detect the RACH of one sector. In MRPO, however, there is a multi-sector cell with several preamble detectors, each dedicated to one sector. Although each CE may look in all sectors and may perform maximum ratio combining, each given CE only considers the RACH messages for its configured sector
FIG. 3 illustrates multiple PRACH detection by a conventional detection process. In a MRPO configuration, it may be possible (due to multi-path propagation, for example) that one PRACH sent by a UE is successfully received in two (or more) sectors by two (or more) CEs. This is shown in FIG. 3. If no precautions are taken, one RACH message detected by multiple CEs and acknowledged individually (by sending an AICH message such as ACK AICH). Furthermore, the received RACH message may be sent by multiple CEs as an independent RACH message towards the RNC, which may pose certain problems.
For example, when several independent RACH messages are transmitted in a PRACH by the corresponding UEs to the network, call-rate related statistics may be worsened. For example, a message from a UE may be received in two separate sectors by two CE's of a Node B which are not in communication with each other. If both CEs detect a RACH preamble from the same UE and inform the controlling RNC (e.g., multiple RACH detection), the RNC may attempt to set-up multiple calls to the UE. However, only one set-up will be successful, the other set-up thus times out. Accordingly, the RNC unnecessarily allocates resources (i.e., 2 channels) when one channel allocation is necessary. Thus statistics such as those which count the number of call drops may increase.
Additionally, and depending on the demodulation loading in each CE, a negative AICH (NACK AICH) could be sent from one CE, and a positive AICH (ACK AICH) could be sent from another CE. This may confuse the UE. Further, if two AICHs are sent to the UE, the received AICH power will be doubled. This may cause the UE to compute an incorrect AICH-to-CPICH power ratio. Accordingly, the conventional processes for parallel RACH detection may not be suitable for an MRPO network configuration.